Recently, significant advances have been obtained to meet the need for Ultra Large Scale Integrated (ULSI) circuits. The development memory devices is continuously driven by new applications. For example, computer interfaces will come to be operated by speech processing or vision processing, and other communication interfaces, all of which require a lot of memory. Read only memory (ROM) devices include ROM cells for coding data and a peripheral controlling devices to control the operation of the cells. Each bit of data is stored in a cell, which is a single n-channel transistor or ROM cell.
Typically, mask ROM includes devices with different threshold voltages. A type of device is formed in an active area and another type of device with a threshold voltage mask is formed in another active area during the process. In MOS transistors for mask ROM, the threshold voltages of the channel regions under the gates are set to the same before data writing. Thereafter, ions are selectively implanted into determined regions to differentiate the threshold voltages thereof for data writing.
One of the methods that involves differentiating the threshold voltages is achieved by the ion implantation of some of the transistor gates. This method raises the threshold voltage of the n-channel device by doping boron with a heavy dose. This method is so called threshold voltage programming. There are two other methods for programming which are field oxide programming and through-hole programming. Each of these programming methods can be implemented at different stages of the process.
In order to achieve the above object, it is known to obtain different threshold voltages by ion implantation through sacrificing oxide into the silicon substrate. For example, in U.S. Pat. No. 5,538,906 to Hitoshi, a method is proposed to form devices with different threshold voltages. However, the high dose boron implantation will result in a lower junction leakage breakdown voltage of the coded MOS and; more importantly, to a very high band-to-band leakage current as mentioned in U.S. Pat. No. 5,683,925. As the gate is decreased down, the oxide must be made thinner in order to provide the same threshold voltage. The higher the dosage for changing the threshold voltage, the more band-to-band current is generated per cell. Thus, Rustom et al. disclose a method of forming ROM without increasing significant band-to-band leakage. The present invention proposes an easier way to manufacture the ROM without additional channel coding implantation.